Conventionally, a radiation noise reduction scheme of pulse width modulation control circuits is proposed from various entities, for example, as disclosed in Japanese patent documents JP-A-H09-204230, and JP-A-2006-174530. More practically, in the first document, the pulse shape of the voltage waveform applied to a gate electrode of the MOS transistor that serves as a PWM driving object is formed in a trapezoid shape or the like for reducing the radiation noise. In the second document, for reducing the radiation noise, the output pulse shape is adjusted to have the synchronization of rising and falling edge timings when two devices are driven by PWM, by devising a phase difference between ON-OFF timings of the two devices.
However, as shown in the first document, when the pulse voltage is formed to have a trapezoid shape, a difficult shape control has to be performed by the PWM control circuit, at the cost of the power loss due to the waveform shaping that leads to heat dissipation. Further, the PWM control of more than three devices is not considered in the disclosure of the second document. That is, the technique in the second document can only be applied to drive only two devices.